Microwave phase shifter



Feb. 1o, 1970 F. w. zlLKosKl MICROWAVE PHASE SHIFTER Filed Aug. 1, 1966|Ll|| Il l .I I I lwlluuhdllrlldcl'lmlllllullllll'll FIGI I9 L 3 F|G.4 l25 g I L- FIG.3

INVENTO I FRANK W. ZIL KI, BY c?. M @Lx HIS ATTORNEY.

United States Patent O 3,495,191 MICROWAVE PHASE SHIFTER Frank W.Zilkoski, Liverpool, N.Y., assignor to General Electric Company, acorporation of New York Filed Aug. 1, 1966, Ser. No. 569,295 Int. Cl.H03h 7/36 U.S. Cl. 333-31 5 `Claims ABSTRACT F THE DISCLOSURE Thisinvention is directed to a microwave phase shifter construction in whichthe magnitude of phase shift in a waveguide is controlled by the depthof insertion of a dielectric card member into the guide. The card isinserted through a slot formed in the waveguide broadwall at the closedend of a reverse bend in the guide, so as to extend through the bendarea and into the straight runs of waveguide adjacent thereto. Manual orautomatic adjustment of the card insertion depth then controls themagnitude of phaseslhift in the guide.

This invention relates to microwave phase shifters and more specificallyto phase shifters of the type providing control of the magnitude ofphase shift by mechanical adjustment of the phase shifter parts.

The phase shifters of this invention afford a level of performancesignificantly improved over that of the conventionally constructed phaseShifters, and at the same time afford a significant simplification ofstructure with consequent improvements in economy and reliability. Amongthe more troublesome of the shortcomings of conventional phase shifterdesigns is their usual requirement for matching devices, couplers orchokes which in addition to undesirable complexity and cost arecharacteristically frequency dependent in their operation and which rasa result tend to limit operation to undesirably narrow bandwidth. Theseand other such common problems as poor linearity or unduly narrow rangeof phase shift adjustment, impedance mismatch and variation with phaseshift adjustment, are substantially alleviated if not wholly eliminatedin the phase Shifters of this invention,

It is accordingly an object of the invention to provide a mechanicallyadjustable phase shifter construction characterized by widebandfrequency capability, Wide range and good linearity of adjustment ofphase shift, and simplicity and economy of construction. Another objectof the invention is the provision of a phase shifter affording inherentmatching with conventional waveguide structures thus avoiding need forfrequency critical matching or coupling devices either within the phaseshifter or to enable its connection to other waveguide. It is also anobject to provide an adjustable phase shifter which presentssubstantially constant impedance irrespective of phase shift adjustment,which does not change wave polarization, and which permits littleleakage even without shielding.

Briefly stated, in one presently preferred embodiment the phase shifterof this invention comprises a length of rectangular section waveguide inthe form of two straight runs disposed parallel to each other andconnected by an 180 E-plane bend, i.e., a reverse bend formed about aline parallel to the waveguide broadwalls. These broadwalls each have aslot therethrough disposed medially of the broadwall and extendinglongitudinally through the area of the bend, with at least the innervbroadwall slot extending beyond the bend and into the straight runs ofwaveguide on either side of the bend. A dielectric card, preferablyhaving its leading edge shaped for optimized reflective matching to thewaveguide, is inserted through the slots in the waveguide broadwalls atthe closed end of the bend so as to extend through the bend ICC area andinto the straight runs of waveguide. Manual or automatic adjustment ofthe depth of insertion of the card then controls the magnitude of phaseshift of waves in the guide, the relationship between phase shift andcard inserted depth being linear through at least part of the availablerange of adjustment.

The invention will be further understood and its various objects,features and advantages more fully appreciated by reference to theappended claims and the following detailed description when read inconjunction with the accompanying drawings, wherein:

FIGURE 1 is a view partially in elevation and partially broken away of aphase shifter in accordance with the invention;

FIGURES 2 and 3 are sectional views taken along the lines 2 2 and 33,respectively, in FIGURE 1; and

FIGURE 4 is a fragmentary elevation of an alternative form of dielectriccard usable in the phase shifters of the invention.

With continued reference to the drawings, wherein like referencenumerals have been used throughout to designate like elements, FIGURE lillustrates the invention in one presently preferred embodiment. Thephase shifter aS shown comprises a metallic body member 11 providingwall means 13 defining a length of rectangular-section waveguide 15opening at its opposite ends 17 and 19 through an end face of the bodymember, Waveguide 15 includes a reverse bend 21 and two straight runs ofguide 23 and 25 interconnected by the bend, these straight runs beingdisposed parallel to each other as best illustrated in FIGURES 1 and 3.

The bend 21 in waveguide 15 is formed about a line parallel to the guidebroadwalls and thus constitutes a E-plane bend with waveguide operationin the normal dominant TELO mode. This bend need not be of thesemicircular form illustrated but may be of any of the various othercurved or stepped configurations known to afford good VSWR and spuriousmode characteristics.

The outer and inner broadwalls of waveguide 15 are slotted as at 27 and29, respectively, with the slots being disposed medially of eachbroadwall and extending longit-udinally along the waveguide through theentire area of the bend 21. The slot 29 through the inner broadwallfurther extends beyond the bend a substantial distance into the twostraight runs of waveguide 23 and 25, thus interconnecting them throughthe slot. It is to be noted that because these longitudinally extendingslots through the broadwalls are medially disposed and relativelynarrow, and because as well known in the art broadwall slots of thisconfiguration do not cause significant loss or other adverse affect onwave energy transmission in the TEU, mode due to the E and H fieldpatterns characteristic of this mode, the slots 27 and 29 do notthemselves noticeably affect waveguide performance.

A dielectric card 31 of quartz or other low-loss dielectric material isinserted through the slot 27 in the outer broadwall at the closed end ofthe bend 21, then through the slot 29 in the inner broadwall to a pointsuch that the forward edge of the card is within the straight runs ofthe guide as shown. Card 31 is generally rectangular in shape and ofwidth just equal to the spacing between the outer broadwalls (theclearance shown being exaggerated for clarity of illustration), so thatthe lateral edges of the card project through the waveguide 15 and intosliding contact with the outer broadwalls. In and out adjustment of thecard then will result in controlled shift of phase of waves within theguide 15, due to the changing effective dielectric constant within thewaveguide resulting from changes in card length therein.

So long as the card is inserted at least sufiiciently far into thewaveguide that its forward edges lie wholly within the straight portions23-25 of the waveguide, the mag- 3 nitude of phase shift will be linearwith the depth of card insertion. This has been shown experimentally andalso may be derived from the elementary relations characteristic ofwaveguides containing dielectric. In such waveguide the guide wavelengthis:

Ag: L Pfff where:

A=free space wavelength Ac--cut-off wavelength of guide (air-filled)e=effective value of dielectric constant in the region of guide underconsideration. In air-filled regions 6:1; in regions containing thedielectric card the effective value of e depends on the thickness of thedielectric card relative to the width of the broadwall.

Since the guide wavelength with the dielectric insert is shorter thanthat of air-filled guide the effective electrical length Le will beincreased with increasing depth of insertion of the dielectric card intothe guide, and phase shift is proportional to the variation in effectivelength of the waveguide resulting from insertion of the card. Waveguideeffective length is given by the expression:

Substituting for AE1 and XE2 there results where as previously noted ecwill lie somewhere between the dielectric constant of air and that ofthe card material, at a value depending on card width and thedimensional relationship between card thickness and broadwall width.

It will be noted that in this expression the Values of all parametersexcept L1 are constant in any given phase shifter design and at givenfrequency, so that the waveguide effective length and thu-s the phaseshift within the guide become linear with L1. Phase shift thus is linearwith depth of card insertion.

The range of adjustment of phase shift may also be seen from thisexpression to be a function of a number of design parameters perhaps thetwo most flexible of which are the depth of insertion (L1) which issubject to no limit except to the extent that dimensional limitationsmay be encountered in the particular installation under consideration.The effective dielectric constant ec also may be varied quite widely, byuse of different dielectric cornpositions for the card and by change ofthe dimensional relationship between card thickness and waveguidebroadwall width.

Card thickness should be kept reasonably small, however, because if thecard and the slots in which it slides are of excessive dimensionundesirably high losses may occur. This results from the fact there iseffectively an upper limit on the product of card thickness and carddielectric constant. This product determines the cutoff frequency of the-slots in which the card slides, and if such cutoff frequency were madeto fall below the operating frequency a dominant mode or modes thencould be ver established in the card and would propagate through theslots with resultant excessive leakage an radiation. Card thickness ofabout 10 percent of broadwall width have in practice been found quitesuitable with conventional dielectric materials, but proportionsdiffering from this value by a factor of two or more in either directionmay be found desirable in certain applications.

For optimizing the reflective matching between airfilled andcard-containing portions of the waveguide, card 31 preferably has itsforward edge tapered as lshown at 33 in FIGURE l or stepped as shown at37 in FIGURE 4. Such edge treatment is alone adequate to hold VSWR andspurious mode generation to acceptably low levels without need for othermatching or coupling devices, thus obviating the problem of frequencysensitivity which the use of such other devices normally involves.

For further reduction of VSWR and spurious mode control two additionalfeatures of construction have been found helpful. As illustrated inFIGURES l and 2, matching at the ends of the inner broadwall slot 29 maybe improved by provision of a taper as at 39, providing more gradualtransition between the unslotted and slotted portions of the waveguide.Also, to reduce undesired transmission of wave energy from one of thestraight runs of guide into the other by propagation through the portionof the dielectric card 31 lying between these waveguide runs, the wallmember defining the inner broadwall slot 29 may be provided with acoating or inset 41 of load material such as a resistive epoxy or lossyfilm. This load material should cover approximately the area shown inFIGURE 1, and serves to dissipate most if not all of any wave energywhich might otherwise propagate in spurious` modes through thedielectric. Similar resistive load means may be provided if necessaryadjacent the outer broadwall slot 29 for dissipating any spurious modewhich might otherwise propagate through the card and out the slot. Suchcontrols are not intended to be effective against any dominant modetransmission through the dielectric card and slot, of course; this isprevented by selection of card thickness and dielectric constant asexplained above so that the cutoff frequency within the slot always isabove the operating frequency.

It'will be noted that since the outer broadwall slot 27 is the onlyexternal opening required in the phase shifter body member for insertionof the dielectric card, a staticr or moving seal can readily beaccomplished in conventional manner where necessary to enablepressurization of the guide. Little if any microwave radiation willoccur through this opening even if left completely Unshielded, againprovided the operating frequency is kept below the cutoff frequency ofthe opening. It will also be noted that in addition to these and thevarious other advantages previously mentioned, the phase Shifters ofthis invention offer the further advantage that they do not affect wavepolarization and that dielectric card movement for phase adjustment doesnot affect impedance.

While in thi-s description of the invention only certain presentlypreferred embodiments have been illustrated and described by way ofexample, many modifications will occur to those skilled in the art andit therefore should be understood that the appended claims are intendedto cover all such modifications as fall within the true spirit and scopeof the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A microwave phase shifter comprising:

wall means defining a rectangular section waveguide including a E-planebend connecting two straight runs of guide, said wall means furtherdefining a longitudinallly extending slot medially disposed in each ofthe waveguide broadwalls in the area of said bend with at least the slotin the inner broadwall extending into said straight runs of guide; and agenerally rectangular dielectric card member, of width substantiallyequal to the spacing between said outer broadwalls, inserted throughsaid slots in the inner and outer broadwalls and extending into saidstraight runs of guide.

2. A microwave phase shifter as defined in claim 1 the area of the bend,said slot in the outer broadwall being of length at least equal to thelength of the bend, and said slot in the inner broadwall being of lengthsuchthat it extends through and beyond said bend and into said straightruns of guide and inter- 5 connects them; and

a generally rectangular dielectric card member projectwherein theforward corners of said dielectric card member are of like shapeoptimized for reflective matching in the guide.

ing through said slots in said inner and outer broadwalls and extendinginto said straight runs of guide, said card being of width at leastequal to the spacing between said outer broadwalls so as to projectthrough the guide in both said straight runs thereof for control of.phase shift therein.

5. A microwave phase shifter as defined in claim 4 r wherein said wallmeans defining said slot in the inner broadwall includes resistive loadmeans disposed adjacent the dielectric ca rd between said straight runsof guide for dissipating energy `which might otherwise propagate throughthe card.

walls with both slots being medially disposed in their respective'broadwalls and extending longitudinally through the area of the bendand with at least the inner broadwall slot extending a substantialdistance beyond the bend and into said straight runs of guide connectingthereto; and 20 a generally rectangular dielectric card member insertedthrough said slots in said inner and outer broad- References CitedUNITED STATES PATENTS walls and extending into said straight runs ofguide, 2,762,973 9/1956 Kallman 324-58 said card being of width at leastequal to the spacing 3,192,492 6./ 1965 Linder 333-31 between said outerbroadwalls so as to project through 25 3,005,168 10/ 1961 Fye 333-31 theguide in both said straight runs thereof and being 2,454,530 11/ 1948Tiley 333--31 movable in said slots so as to enable adjustment of2,630,492 3/1953 Muchmore 333-31 phase shift by adjustment of cardinsertion depth. 3,316,509 4/ 1967 Ayer et al. 333-31 4. A microwavephase shifter comprising:

wall means defining a rectangular section waveguide in the form of a 180E-plane bend connecting two straight runs of guide, said wall meansfurther defining a longitudinally extending slot medially disposed ineach of the waveguide broadwalls through 30 HERMAN KARL SAALBACH,Primary Examiner C. BARAFF, Assistant Examiner j U.S. Cl. X.R. S33-34,84

